ABSTRACT Radiotherapy has become an increasingly effective technique to treat patients affected by primary and metastatic liver cancers, especially after the advent of stereotactic body radiotherapy (SBRT). SBRT delivers a large and focused radiation dose to liver tumors and achieves superior local control and survival. However, for current liver SBRT, a volume usually much larger than the actual tumor (with treatment margins up to 15 mm beyond the tumor boundary) is treated, to account for tumor localization uncertainties under the cone-beam computed tomography (CBCT) image guidance. Such a large treatment volume incurs more radiation to normal liver tissues and organs-at-risks and raises the concern of normal tissue toxicity, especially for patients with liver cirrhosis and a limited healthy liver tissue reserve. It also prevents further dose escalation to maximize the effectiveness of SBRT. Currently, there lacks a reliable technique to accurately localize liver tumors by CBCT, mostly due to the respiration-induced liver motion and the low contrast of liver tumors against the normal liver tissues. In response to PAR-19-158, we propose to develop a biomechanical modeling-guided 4DCBCT technique (Bio- 4DCBCT), which generates 4DCBCT images to capture the liver tumor motion for accurate 4D localization. The Bio-4DCBCT incorporates liver biomechanical modeling into the 4DCBCT generation process, which can substantially improve the localization accuracy of CBCT at low-contrast regions. The goal of this study is to develop, optimize and validate the Bio-4DCBCT technique to achieve ~ 2 mm tumor localization accuracy. In this study we will pursue three specific aims: SA1. Optimize Bio-4DCBCT through a retrospective patient study. SA2. Evaluate Bio-4DCBCT by developing a motion-enabled, deformable, physical liver phantom. SA3. Evaluate Bio-4DCBCT through a prospective clinical study by comparing Bio-4DCBCT with the current clinically-applied techniques. The innovation of the project is the Bio-4DCBCT technique and its application for accurate 4D liver tumor localization, which leads to substantial treatment volume/margin reduction to achieve safer, more viable and more effective liver SBRT. The Bio-4DCBCT technique is developed on the conventional CBCT system under standard image acquisition protocols, which will be readily applicable in radiotherapy clinics worldwide without hardware upgrade or image acquisition protocol change. The successful clinical translation of this technology is expected to create a favorable shift in current tumor localization paradigms, not only for liver but also for other tumor sites where low tissue contrast adversely affects the radiotherapy treatment accuracy.